The present invention generally relates to electrography and more particularly, to a magnetic brush developing apparatus for use in an electrographic copying machine and the like.
Recently, as one form of the magnetic brush developing apparatus of the above described type, there has been put into practical use an arrangement which includes a developing sleeve confronting a surface of an electrostatic latent image support member and driven for rotation in a direction opposite to the direction of transportation of a developing material, and a magnetic roller rotatably provided within said developing sleeve and driven for rotation at high speed in the same direction as the direction of rotation of said developing sleeve for forming a magnetic brush by magnetically attracting the developing material onto the outer peripheral surface of the developing sleeve through the magnetic force of the magnetic roller, and the magnetic brush thus formed is transported in a direction opposite to said rotating direction, by the rotation of the magnetic roller so as to be brought into rubbing contact with the surface of the electrostatic latent image support member for developing the electrostatic latent image formed thereon.
In the above case, although the developing material is transported, on the whole, in the direction as described above, part of the developing material in the vicinity of the tip portion of the magnetic brush is displaced in a direction opposite to the direction of rotation of the developing sleeve through magnetic action based on the rotation of the magnetic roller, while part of the developing material close to the root portion of the magnetic brush is moved in the rotating direction of the developing sleeve, based on the rotation of said developing sleeve, i.e. in the direction opposite to the direction of transportation of the developing material. During the transportation, both parts of the developing material as described above are caused to stir each other and there is an exchanging of developing material therebetween. More specifically, as a result of the above agitation, the direction of movement of the developing material to be displaced in the direction opposite to the direction of transportation of the developing material is changed to the opposite direction, i.e. toward the developing region, and thus, part of the developing material equivalent in amount to the amount of the developing material the direction of movement of which has been changed (the above part of the developing material is the part being displaced in the direction of transportation referred to above) has its direction of movement reversed. In the magnetic brush developing device as described above and put into practical use at present, since the point of supply of developing material to the outer peripheral surface of the developing sleeve is spaced a comparatively long distance from the developing region, the distance of transportation of the developing material is sufficiently long to allow exchange between both parts of the developing material as described earlier at a considerable rate.
However, according to the experiments carried out by the present inventors, it has been found that the exchanging of the developing material as described above tends to give rise to lowering of toner concentration in the developing material at the developing region, with a consequent reduction of density of the copied images (i.e. developed image density). More specifically, in the developing region, toner is consumed during development of the electrostatic latent image, resulting in reduction of the toner concentration in the developing material, and part of the developing material with the lower toner concentration is moved in the direction opposite to the direction of transportation of the developing material on the whole, following rotation of the developing sleeve as described earlier so as to be exchanged with the developing material newly supplied onto the outer peripheral surface of the developing sleeve and coming into the developing region. As a result, toner concentration in the developing material being transported is substantially lowered, and thus, on the whole, developing material having a substantially lowered toner concentration as compared with the supplied developing material is transported to the developing region with a consequent reduction in the image density. The phenomenon as described above is especially noticeable when a so-called entirely black copy is made, i.e. when a solid electrostatic latent image is developed.